Automatic transmission systems are widely used in automotive vehicles, and the shifting operation of the transmission is automatically carried out according to various operating conditions of the vehicle in such transmission systems. Typically, as disclosed in Japanese patent laid open publication (kokai) No. 1-295062, the upshift of the transmission takes place according to a certain condition which depends on the vehicle speed and the throttle opening representing the load of the engine, and the speed at which the upshift takes place is raised as the load on the engine is increased. The condition for the upshift from each of the speed stages of the transmission system is indicated by the solid lines in FIG. 3 .
In a vehicle equipped with such an automatic transmission system having such operation characteristics, the operator of the vehicle typically steps on the accelerator pedal by a certain amount in start-off, and maintains the accelerator pedal at a fixed position once a desired acceleration is achieved or a desired speed has been reached. The upshift of the transmission system typically takes place after the accelerator pedal is kept substantially stationary, and the speed stage of the transmission is shifted automatically from the first stage to the second, third and the fourth stages in a sequential manner. At such a time, even when the vehicle operator tries to keep the pedal position stationary, if there is a slight changes in the position of the accelerator pedal, the throttle opening changes accordingly as indicated by the broken line in FIG. 3. Thus, as indicated by white circles at vehicles speeds Vo1 to Vo3, the upshifts take place at uneven intervals. However, such an upshift pattern causes an disagreeable impression on the vehicle operator.